The long-term goal of this work is to provide a scientific basis for treating crouch gait, the most common movement abnormality among persons with spastic diplegia. Crouch gait is characterized by persistent flexion of the knee, which is usually accompanied by excessive flexion, adduction and internal rotation of the hip. It is an extremely inefficient means of locomotion; if not corrected it often leads to joint degeneration. Although surgeries such as: (i) lengthening of the hamstrings; (ii) derotation of the femur; and (iii) transfer of the rectus femoris are frequently performed in an attempt to improve musculoskeletal function, the outcomes of these surgeries are unpredictable and often unsatisfactory, leaving patients with serious, life-long physical limitations. The purpose of this work is to develop biomechanical models that explain the causes of crouch gait and predict the functional consequences of commonly used surgical treatments. Aim #1 will examine the causes of excessive knee flexion. Based on preliminary data, it is hypothesized that many persons with crouch gait have hamstrings that are of adequate length for normal walking. This hypothesis will be tested by estimating the lengths of the hamstrings during normal and crouch gait using detailed musculoskeletal models in conjunction with measured three-dimensional joint kinematics. This analysis may enable a more rational basis to be developed for deciding who should, and should not, have hamstring lengthening. Aim #2 will examine the causes and corrections of excessive internal rotation of the hip. The rotational moment arms of the muscles crossing the hip will be determined based on quantitative anatomical studies and three dimensional computer models constructed from magnetic resonance images. The computer models will be used to analyze the effects of femoral deformities and derotational osteotomies on the rotational function of the muscles. This analysis is intended to reveal the best means to restore normal limb rotation. Aim #3 will compare measurements taken from subjects after transfer of the rectus femoris to a computer simulation of the surgery. This study will also determine if subjects use the rectus femoris as a knee flexor after it is transferred behind the knee. It is suggested by the applicant that although multijoint movement abnormalities such as crouch gait are exceptionally complex, the development of quantitative descriptions of muscle function in normal, deformed, and surgically altered limbs, as proposed here, is an important and necessary first step toward designing more effective treatments.